private: Faster channel switching

This enables faster channel switching, by not parsing the stream info for
PVR live streams completely beforehand. This took several seconds and
slowed down the channel switching. The new method reads the stream info
when it's available.

From:
github.com/FernetMenta/xbmc/commit/bf753ebec5f231bc7d85b6404c0841cf0307422d
github.com/margro/xbmc/commit/ef7442e5797b0eed22d9eee04e0571ac1d0453c9

Credits: FernetMenta, margro